Rare earth magnet manufacturers have continued consecutive improvements in composition and development of more efficient preparation methods. Nowadays, it becomes possible to produce high-performance magnets having a (BH)max of 50 MGOe or greater and a coercive force of 30 kOe or greater. They are used in a wider variety of applications including not only parts in consumer appliances and computer-related equipment such as voice coil motors (VCM) and pickup sensors for CD and DVD, and medical equipment like MRI as often found in the past, but also electric and electronic parts such as motors and sensors.
In the case of permanent magnet motors, for example, economical ferrite magnets were used in the past, but have been increasingly replaced by rare earth magnets to meet the current demand for motors with reduced size and increased efficiency. The rare earth magnets on general use include Sm—Co magnets and Nd—Fe—B magnets. The Sm—Co magnets experience little changes with temperature of magnetic properties due to high Curie temperature, and eliminate a need for surface treatment due to corrosion resistance. However, they are very expensive because of their composition with a high cobalt content. On the other hand, the Nd—Fe—B magnets have the highest saturation magnetization among permanent magnets and are inexpensive because the major component is inexpensive iron. The Nd—Fe—B magnets, however, experience substantial changes with temperature of magnetic properties due to low Curie temperature, and lack heat resistance. Since they also have poor corrosion resistance, an appropriate surface treatment must be carried out in a certain application.
Rare earth magnets have a resistivity of about 150 μΩ-cm which is lower by two orders than that of ferrite magnets. Therefore, a problem arises when rare earth magnets are used in motors. Since a varying magnetic field is applied across the magnet, eddy current is created by electromagnetic induction. By the Joule heat due to eddy current flow, the permanent magnet generates heat. As the temperature of permanent magnet is elevated, magnetic properties degrade, particularly in the case of Nd—Fe—B sintered magnets having noticeable changes with temperature of magnetic properties. As a result, the efficiency of the motor deteriorates. This deterioration is referred to as eddy current loss.
There have been considered and proposed several countermeasures against such deterioration including    (1) to increase the coercive force of a magnet,    (2) to divide a magnet into segments in a magnetization direction,    (3) to provide an insulating layer within the magnet interior, and    (4) to increase the resistivity of a magnet.
In method (1), heavy rare earth elements such as Dy substitute for part of Nd—Fe—B to enhance the magnetocrystalline anisotropy and coercive force. The heavy rare earth elements used for partial substitution are short in resource and expensive. Undesirably, this eventually increases the cost of magnet unit.
In method (2) of dividing a magnet into segments, the heat value generated is controlled by reducing the area across which the magnetic flux penetrates or by optimizing the aspect ratio of the area across which the magnetic flux penetrates. The heat value can be further reduced by increasing the number of divisions, which undesirably increases the manufacturing cost.
Method (3) is effective when the external magnetic field varies parallel to the magnetization direction of the magnet, but not effective in actual motors where the varying direction of the external magnetic field is not fixed.
In method (4), the resistivity of a magnet at room temperature is increased by adding an insulating phase. Depending on a particular insulating material selected, densification is difficult, so that magnetic properties and corrosion resistance are deteriorated. A special sintering technique must be employed for achieving densification.
Reference should be made to JP-A 2003-070214, JP-A 2001-068317, JP-A 2002-064010, JP-A 10-163055, and JP-A 2003-022905.